A direct iron-making process for making reduced iron by direct reduction of an iron oxide source such as iron ore using a carbonaceous substance or a reducing gas has long been known. Extensive research has been conducted as to the specifics of the reducing process and continuous reduction equipment.
For example, Japanese Unexamined Patent Application Publication No. 11-337264 discloses a rotary hearth that allows efficient continuous production of reduced iron, in which, during reduction by heating of green pellets prepared by solidifying a mixture of an iron oxide source such as steelmaking dust or fine ore and a carbonaceous substance using a binder, explosions which occur when undried green pellets are rapidly heated are prevented due to installation of a preheating zone.
In the technology, including the above-described technology, for making metallic iron by heating and reducing compacts containing an iron oxide source and a reductant, a considerable amount of a slag component becomes mixed in the resulting metallic iron due to the use of the iron ore or the like. In particular, in a method for making sponge metallic iron, the Fe purity is drastically low because the separation of the slag component that became mixed in the metallic iron is difficult. Thus, a preliminary treatment for removing this considerable amount of slag component is required when these materials are used as an iron source. Moreover, nearly all of the metallic iron obtained by a known direct iron-making process is sponge-shaped, and thus the handling thereof as an iron source is difficult since such metallic iron is fragile. In order to actually use such metallic iron as a material for making iron, steel, or alloy steel, a process such as a secondary process to make briquettes therefrom is required, and the expenses for additional equipment therefor are considerable.
Japanese Unexamined Patent Application Publication No. 9-256017 discloses a method for making metallic iron nuggets having a high metallization ratio, the method including heating and reducing compacts containing iron oxide and a carbonaceous reductant until a metallic iron sheath is formed and substantially no iron oxide is present in the inner portion while forming nuggets of the produced slag in the inner portion, continuing heating so as to allow the slag inside to flow outside of the metallic iron sheath so as to separate the slag, and further performing heating so as to melt the metallic iron sheath.
In the known processes, including these conventional techniques, for making metallic iron nuggets, no technology capable of efficiently making metallic iron having a diameter within a predetermined range while fully considering the quality and handling convenience of materials for making iron, steel, or iron alloy has been established. As for the purity of the metallic iron nuggets, although high-purity metallic iron nuggets with a low contaminant content are naturally preferred, no specific idea for specifying the optimum carbon content in the metallic iron nuggets used as the material for iron making and steelmaking has been formulated. Moreover, no specific manufacturing technology for controlling the carbon content within a predetermined range has been established.
Furthermore, when metallic iron is made by reducing iron oxide such as ore, coke or a coal powder is generally used as the reductant. However, these reductants normally have a high sulfur (S) content. Since the reductant becomes mixed in the metallic iron produced, the resulting metallic iron nuggets normally have a high S content. Accordingly, the metallic iron nuggets must be subjected to desulfurization before they are actually used as the material for making iron or steel. This is also one of the main reasons for the degradation in quality of the metallic iron nuggets.
Accordingly, in order to make metallic iron nuggets of high value by a reducing-melt process, it is not sufficient to merely hope to increase the purity. A technology that can reliably make metallic iron, in which the contaminant content, such as a sulfur content, is specified and the size of which is optimized in view of production possibility and handling quality, the technology also being capable of satisfying the demands in the market such as a greater flexibility in the choice of material for making iron, steel, or various alloy steels, and reduction of the cost required for making iron or steel using, for example, an electric furnace, is required to be established.
The present invention is developed based on the above-described background. An object of the present invention is to provide metallic iron nuggets of stable quality that have an optimum size in view of the overall production possibility and handling quality as an iron source, and in which the contaminant content of the metallic iron nuggets, such as carbon and sulfur contents, is specified. The metallic iron nuggets of the present invention can thus satisfy the demands in the market such as a greater flexibility in the choice of material for making metallic iron and a reduction of the cost required for making iron or steel using, for example, an electric furnace.